This invention relates to an improved foundry binder composition for ambient temperature curing, and especially for alkaline aggregates such as reclaimed sand, Olivin sand, alumina sand, etc.
Conventional foundry binders for ambient temperature curing are classified under two types; one is inorganic binders, typified as water glass, and the other is organic binders, such as the acid curing type and urethane forming type. While inorganic binders scarcely impair the environment of workshop because then emit few toxic gases when used, the molds obtained therefrom are poor in the shake-out property after pouring. Thus, difficulties in the shake-out property reduce the availability of utilization of the reclaimed sand.
On the other hand for organic binders, acid-curing binders made of phenolic resins and furan resins require large quantity of strong acids at the curing thereof, and this causes corrosion on the equipment, and an irritant gas at pouring is emitted, although these binders are excellent in the shake-out property of the molds therefrom.
A method of ambient-temperature-curing is known for producing foundry molds with a binder system comprising the reaction of polyols of a phenolic resin or an oil modified alkyd resin with a polyisocyanate in the presence of liquid amines or metal naphthenates, and this method is useful in a large dimensional molding with compact equipment. However, since this method necessitates a long curing time, it cannot be employed for a commercial stage production of the molds.
A method for commercially producing foundry molds under ambient-temperature-curing is called "gas-curing mold forming process", which allows a high productivity with a rapid mold forming cycle by passing gaseous amines into a mixture of foundry aggregates in the presence of both a phenolic resin as a polyol component and a polyisocyanate. This system is widely used into the foundry industries because of its energy-saving. However, when highly alkaline sand typified as reclaimed sand, Olivin sand, alumina sand, etc. having the pH value of 7 or more, is employed as foundry aggregates, even in coating thereof with a binder, the obtained coated sand is often so degraded that it is useless in mold forming because a prior reaction of the phenolic resin with the liquid polyisocyanate of binder components. This occurs by means of a catalytic behavior of said alkaline composition in silica sand.
This is an accelerated urethane forming reaction affecting flow of obtained coated sand, and this fails in smoothness of mold surface as well as strength of the molds obtained therefrom. Therefore, cast articles made of said molds are likely to have defects such as an irregular surface, occlusion of sand thereinto, etc. Said degradation often occurs during a hot-humid summer season, at excessively elevated temperature of sand, or in a high content of said alkaline composition in silica sand. This results in the fact that said urethane forming reaction depends on temperature as well as an existing quantity of catalytic substances. So, to suppress said urethane forming reaction, a method is known to incorporate an acidic substance into a phenolic resin component of binders for neutralizing said alkaline composition of silica sand. However, when said acidic substance is short of the neutralizing equivalent, it is not effective. On the contrary, when it is excessive, said phenolic resin component spontaneously give rise to a condensation reaction which enables the component to be useless as a binder. There is a method by incorporating said acidic substance into a liquid polyisocyanate component of binders, however, in the method the activated hydrogen of said acidic substance reacts with isocyanate groups which results not only in a poor reactivity thereafter, but, an inert precipitation occurs at mixing with a phenolic resin component.
A method is also known that a small quantity of said acidic substance is directly incorporated into a mixer when coating sand with both a phenolic resin and a liquid polyisocyanate component. However, this method requires additional equipment, and the operation therefor becomes complicated. Furthermore, this method is essentially difficult in mixing as dispersing small quantities of said acidic substance homogeneously in a large quantity of formulation is not a practical solution.
The present invention results from an investigation to overcome these drawbacks. The inventors have found that the incorporation of a substance that will emit an acid upon hydrolysis will neutralize the alkaline components present.